1. Field of the Invention
The present invention relates to a lamp illumination adjusting apparatus for automobiles, which is improved to decrease the heat generation and to enable the compact structure.
2. Description of the Prior Art
In an example of a lamp illumination adjusting means in the prior art, series connection of a lamp and a power transistor is interposed between a power source terminal and the ground, and impedance of the power transistor is adjusted, thereby amount of driving current flowing through the lamp is adjusted so as to adjust the illumination. In this illumination adjusting means, however, since large power loss is produced in the power transistor and the heat generation is much, a large heat radiation fin is required and the apparatus cannot be made compact. Moreover, value of the driving current and the intensity of illumination of the lamp are in non-linear relation and therefore means to correct this relation is required.
Consequently, a lamp illumination adjusting apparatus has been proposed where a power transistor is turned on or off by PWM (Pulse Width Modulation) control and the power loss is made little.
FIG. 4 is a circuit diagram of an example of a lamp illumination adjusting apparatus by PWM control in the prior art.
In FIG. 4, series connection of a lamp 2 and a power transistor 3 is interposed between a power source terminal 1 and the ground. Base of the power transistor 3 is supplied with bias voltage by resistors 4, 5, and grounded through a transistor 6. Base of the transistor 6 is supplied with pulse signals outputted from a timer 7. The timer 7 adjusts period of the pulse signals by adjusting a slidable terminal of a variable resistor 8 by manual operation.
In such constitution, if pulse signal is outputted from the timer 7 and the transistor 6 is turned on, the power transistor 3 is turned off and the driving current of the lamp 2 is interrupted. While pulse signal is not outputted from the timer 7, the transistor 6 is turned off and the power transistor 3 is turned on by bias voltage set by the resistors 4, 5 thereby the driving current flows through the lamp 2. In this state, period of the pulse signal outputted from the timer 7 is adjusted, thereby duty of the power transistor 3 is adjusted so as to adjust the average intensity of illumination of the lamp 2.
In this constitution, the lamp 2 is PWM controlled, thereby the power loss due to the power transistor 3 is decreased and the heat generation is decreased, thus the power transistor of small capacity may be used and a heat radiation fin may be of small size and the apparatus can be made compact. Moreover, since the average intensity of illumination of the lamp 2 is adjusted by adjusting proportion of lighting and putting-out of the lamp, period of the pulse signals and the average intensity of illumination are in linear relation.
In the above-mentioned lamp illumination adjusting apparatus of the prior art, however, current flows always through the resistor 4 which sets bias voltage to the power transistor 3 thereby the power loss is produced. Consequently, an element with high wattage must be used as the resistor 4 and therefore the element becomes large size. Moreover, in order to radiate the heat of the resistor 4, the lamp illumination adjusting apparatus cannot be incorporated within a closed cabinet. Particularly, in use within an atmosphere including a large amount of dust or mist of automobiles, it becomes a serious problem that the apparatus as a whole cannot be incorporated within a closed cabinet.
Moreover, since the PWM control is performed in response to pulse signal outputted from the timer 7, the lamp 2 is put out while the pulse signal is outputted, thereby the duty cannot be adjusted in wide range of 0.about.100%.